1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly to an exhaust gas purifying apparatus having a particulate filter in an exhaust system, wherein the exhaust gas purifying apparatus performs an abnormality determination in regeneration control that is executed while burning particulates accumulated in the particulate filter.
2. Description of the Related Art
Japanese Patent Laid-open No. 2002-70619 (JP-'619) discloses a method for determining an abnormality in the regeneration control of a diesel particulate filter (hereinafter referred to as “DPF”). According to the method disclosed in JP-'619, two oxygen concentration sensors are provided, respectively, upstream and downstream of the DPF in the exhaust system, and an upstream side oxygen concentration and a downstream side oxygen concentration are detected. Further, an oxygen concentration change rate, which is a rate of change in a difference between the upstream side oxygen concentration and the downstream side oxygen concentration (concentration difference), is calculated. It is determined that an abnormal temperature rise of the DPF occurs when the oxygen concentration change rate is greater than a predetermined change rate for a predetermined time period or longer.
The method disclosed in JP-'619 is used with a control system wherein a temperature rise in the DPF is estimated according to the oxygen concentration change rate, and an intake air flow rate of the engine is adjusted according to the estimated temperature increase.
However, in a control system wherein the above-described adjustment of the intake air flow rate is not performed, it is necessary to determine if an abnormality in regeneration control has occurred wherein an amount of oxygen flowing into the DPF is insufficient and the accumulated particulates can not sufficiently burn during execution of the regeneration control.
Further, when performing regeneration control of the particulate filter, the injected fuel is used for burning the particulates. If the fuel supply state is abnormal (especially if an excessive amount of fuel is supplied), too much oxygen is consumed by the excessive amount of fuel before the oxygen can reach the particulate filter. Consequently, the amount of oxygen is insufficient for burning the particulates trapped in the particulate filter. Therefore, it is also necessary to determine that such an abnormality is present.